1. Field of the Invention
The present invention relates to a method of fabricating semiconductor devices, and more particularly, to a method of forming a metal nitride film by chemical vapor deposition (CVD) where a metal source and a nitrogen source are used as a precursor, and a method of forming a metal contact of a semiconductor device using the above method.
2. Description of the Related Art
A barrier metal layer, which prevents mutual diffusion or chemical reaction between different materials, is indispensable to stabilize the contact interfaces of semiconductor devices. In general, a metal nitride such as TiN, TaN or WN has been widely used as the barrier metal layer of semiconductor devices. Here, TiN is a representative example among the above metal nitrides.
However, when the metal nitride film such as TiN is fabricated by sputtering, its application to highly integrated semiconductor devices is not appropriate, due to low step coverage. For an example, FIGS. 9A and 9B show the cross-section of a via contact for connection between metal wiring. FIGS. 9A and 9B show a simple via contact and an anchor via contact, respectively. The formation processes thereof are as follows. A first metal layer 30 composed of aluminum (Al) is formed on a semiconductor substrate 20. A TiN film 40 is formed as a capping film on the resultant structure by sputtering, and then an interlayer insulative film 50 or 51 is deposited. A contact hole is formed by etching the interlayer insulative film 50 or 51 on the first metal layer 30. In FIG. 9B, the step of forming an anchor A by wet etching is added. After Ti as an adhesive layer and TiN 60 or 61 as a barrier metal layer is deposited, a tungsten (W) plug 70 or 71 is formed to fill the contact hole, by CVD. Thereafter, tungsten at the upper portion is removed by chemical mechanical polishing or etch-back, and then a second metal layer is deposited on the resultant structure, thereby completing the connection between metal wiring. However, this last step is not shown.
Here, in a conventional method, the TiN film 60 or 61, being the barrier metal layer, is deposited by sputtering, with inferior step coverage. Here, the thickness of a TiN film on the bottom, corner and anchor A of the contact hole is reduced, with an increase in the aspect ratio of the via. Accordingly, at a thin portion, Ti or Al combines with fluorine remaining in tungsten source gas WF.sub.6 during tungsten deposition being a subsequent process, and thus an insulative film X forms of TiF.sub.x or AlF.sub.x leading to a contact failure.
When the contact failure is avoided by increasing the deposition time to increase the thickness of the TiN film 60 or 61, the thickness of the TiN film increases only at the upper portion of the contact hole, and the upper portion of the contact hole is narrowed or blocked. Thus, voids are likely to be generated upon subsequent tungsten deposition. A process with improved step coverage is required to apply TiN to a contact with a high aspect ratio. Accordingly, a process for fabricating a metal nitride film using CVD (hereinafter called a CVD-metal nitride film) has been developed as a next generation process.
A general process for forming a CVD-metal nitride film uses a metal source containing chlorine (Cl), e.g., a precursor such as titanium chloride TiCI.sub.4. The CVD-metal nitride film using TiCI.sub.4 as the precursor has a high step coverage of 95% or higher and is quickly deposited, but Cl remains in the metal nitride film as impurities. The Cl remaining as impurities in the metal nitride film causes corrosion of metal wiring such as Al and increases resistivity. Thus, the Cl content in the metal nitride film must be reduced and the resistivity must be lowered, by deposition at high temperature. That is, in the CVD-metal nitride film process using the metal source such as TiCl.sub.4, a deposition temperature of at least 675.degree. C. required to obtain resistivity of 200 .mu..OMEGA.-cm or less. However, a deposition temperature of 600.degree. C. or more exceeds thermal budget and thermal stress limits which an underlayer can withstand. In particular, when the metal nitride film is deposited on an Si contact or a via contact with an Al underlayer, a deposition temperature of 480.degree. C. or lower is required, so that a high temperature CVD-metal nitride film process cannot be used.
A low temperature deposition CVD-metal nitride film process is possible, by adding MH (methylhydrazine, (CH.sub.3)HNNH.sub.2) to the metal source such as TICl.sub.4, but this method has a defect in that step coverage is decreased to 70% or lower.
Another method capable of low temperature deposition is to form a MOCVD-metal nitride film using a metalorganic precursor such as TDEAT (tetrakis diethylamino Ti, Ti(N(CH.sub.2 CH.sub.3).sub.2).sub.4), or TDMAT (tetrakis dimethylamino Ti, Ti(N(CH.sub.3).sub.2).sub.4). The MOCVD-metal nitride film has no problems due to Cl and can be deposited at low temperature. However, the MOCVD-metal nitride film contains a lot of carbon (C) as impurities, giving high resistivity, and has inferior step coverage of 70% or less.
A method of forming a metal nitride film using atomic layer epitaxy (ALE) has been tried as an alternative to deposition, in order to overcome the problems due to Cl. However, the ALE grows the metal nitride film in units of an atomic layer using only chemical adsorption, and the deposition speed (0.25 .ANG./cycle or less) is too slow to apply the ALE to mass production.